unequal probes used as voltage and floating ones and vice versa, the better the voltage probe conducts the smaller the fluctuations of R$_{4t}$ . Importantly, if the dephasing probe model is used, dephasing channels should be in- cluded not into the real, but into the floating probes. is anti-symmetric, and any loop contributes equally to both terms. Consequently, WL corrections are canceled, and they appear only if 〈 R$_{4t}$ 〉 ̸ = 0, e.g. in a quasi-1d dis- ordered structure [30] with a special probe arrangement. Similarly, the TRS-breaking does not affect the fluc- tuations of R$_{4t}$ as shown by Eq. (3), in contrast with Eq. (4) for R$_{2t}$ . Indeed, inverting time we must invert magnetic flux Φ and swap the current- with voltage- probes. The ensuing symmetry R$_{2t}$ (Φ) = R$_{2t}$ ( − Φ) gives ∂R$_{2t}$/∂ Φ = 0 at Φ = 0, but there is no such a restric- tion on R$_{4t}$ (Φ). Fluctuations of R$_{2t}$ (Φ) are analogous to a perturbation on a string with a free end, where the perturbation maximum corresponds to the maximum in R$_{2t}$ -fluctuations at Φ = 0. On the other hand, R$_{4t}$ (Φ) is analogous to an infinite string, so that zero field is as good as any other: neither TRS-breaking effects like WL in Eq. (2) or 2 /β -coefficient of Eq. (4), nor ”quenching” (diminished sensitivity to Φ at Φ = 0) appear for R$_{4t}$ in chaotic dots [28]. In addition, in contrast to the classi- cal four-terminal scheme [3], the properties of source and drain are never fully eliminated: a measurement affects the sample and so the mesoscopic fluctuations in R$_{4t}$ do depend on contacts. Importantly, in the experiment [18] the voltage probes are invasive. AC setup, ω = 0 . Now we generalize our dc re- sults to finite frequency ω to consider dynamics of resis- tances. The screened conductance g s ω in Eq. (6) becomes complex and depends not only on τ$_{d}$ [34], but also on τ$_{RC}$ = C$_{µ}$/ tr Γ. Here C$_{µ}$ is the electro-chemical capaci- tance, which relates the charge of the dot to the chem- ical potentials, it is defined by the Coulomb interaction strength, 1 /C$_{µ}$ ≡ 1 /C +Δ /ν$_{s}$ [25]. A frequency ∼ 1 /τ$_{RC}$ , when the capacitor and the contacts conduct similarly, is then a natural characteristic of a two-terminal mea- surement. The finite-frequency correlations and WL of conductance were considered only for ballistic QPCs at T, ω → 0. Quantum effects in ac transport depended only on ωτ$_{d}$ at low frequencies [21], but at higher ω they start to depend on ωτ$_{RC}$ as well. In contrast, for R$_{2t}$ our Eq. (2) shows that at any frequency the WL is indepen- dent of τ$_{RC}$ ; this is compatible with τ$_{RC}$ -dependent WL in conductance [21], because τ$_{RC}$ is eliminated by the matrix inversion. The reason is that WL only increases the return probability, but does not redistribute charge. Low-frequency conductance correlations can be gen- eralized to arbitrary frequencies ω, ω ′ using parameter z ≡ − iωτ$_{RC}$/ (1 − iωτ$_{RC}$ ) and z ′ introduced similarly, and considering ballistic QPCs with n$_{i}$ = N$_{i}$/N for sim- plicity. For β = 2 we find the correlations, For the non-ideal contact coupling a tunneling limit for the voltage probes is often taken, tr Γ$_{v}$ ≪ 1; in the opposite limit, tr Γ$_{v}$ ≫ 1, UCF presents a complicated 6-th order polynomial of Γ$_{j}$ [20]. In contrast, for a fixed set { Γ$_{j}$ } the fluctuations of R$_{4t}$ given by Eq. (3) can be expressed only in terms of conductance tr Γ$_{j}$ and Fano factor F $_{j}$, the ratio of shot noise to current [31], of each QPC. The result of a separate averaging over mesoscopic contact j , tr Γ 2 $_{j}$/ tr $^{2}$Γ$_{j}$ → (1 − 〈F j 〉 ) / 〈 tr Γ$_{j}$ 〉 , is propor- tional to the QPC resistance with a numerical prefactor: it ranges from 1 for ballistic to 2/3 for a short diffusive wire, if disorder is increased; for a wide tunneling QPC it is equal to 1 if Γ$_{j}$ ≪ 1 are the same, and 1 / 2 for a dirty interface [32]. We observe that, generally, a poorer coupling increases fluctuations of R$_{4t}$ . and for β = 1 the same r.h.s. with k ↔ l should be added. Results of Ref. [21] are reproduced by differen- tiation ∂$_{ω}$$_{(}$$_{ω}$ ′ ) of Eq. (7) at ω, ω ′ = 0. Using Eq. (7) we find that fluctuations of R$_{2t}$ in a dot with ballistic probes, N = N$_{1}$ + N$_{2}$ + N$_{f}$ , given by Eq. (4) manifestly depend on both τ$_{d}$ and τ$_{RC}$ . Only the four-probe scheme can show if ωτ$_{RC}$ is an intrinsic scale of the dot itself, or if it comes from the chosen two-terminal measurement scheme. In other words, does the frequency-scale 1 /τ$_{RC}$ survive in resistance statistics if the role of contacts is minimized? Equation (3) for R$_{4t}$ does not depend on τ$_{RC}$ , and we conclude that this scale is extrinsic and can be under- stood as a contact property. Since Gaussian R$_{4t}$ is fully characterized by Eqs. (1,3), we conclude that the quan- tum transport at arbitrary frequency ω is insensitive to screening only in the four-terminal scheme. Experimental relevance. Recently, Lerescu et al. [18] measured statistics of R$_{4t}$ in dc-biased ballistic dots and compared with numerical data for N$_{f}$ = 0. The rms R$_{4t}$ qualitatively agreed with the numerics, but was about 20 Often conductors have more contacts then needed for a four-terminal measurement. A floating (unused) con- tact f does not to draw any current from the sample, but leads to decoherence [33], its coupling is related to the inelastic scattering time τ$_{in}$ = h/ Δtr Γ$_{f}$ [19]. Data are often interpreted as an addition of several fictitious dephasing channels to the real contacts. However, here it is important to distinguish between a floating probe and a contact used for transport measurements. Simi- larly to Ref. 19, we find that a floating probe decreases fluctuations of R$_{4t}$ by increasing the total coupling tr Γ of the dot to reservoirs. Due to the zero-dimensional ge- ometry of the dot, they are independent of the number of such probes, and only the total tr Γ$_{f}$ is relevant. The voltage probes used to define R$_{4t}$ and floating probes are not equivalent, an increase of the former decreases fluctuations stronger. Indeed, in two measurements with 〈 g s $_{ij,ω}$g s kl,ω ′ 〉 ν 2 $_{s}$n$_{i}$ n$_{j}$n$_{k}$n$_{l}$ = ∫ dτ (2 π $^{2}$T ) $^{2}$e − τ /τ d ( e iωτ − 1)(1 − e iω $^{$^{′}$τ}$) τ$_{d}$ωω $^{′}$sinh $^{2}$(2 π $^{2}$T τ ) × [ ( δ$_{ik}$/n$_{k}$ − 1 + zz $^{′}$)( δ$_{jl}$/n$_{l}$ − 1 + zz $^{′}$) 1 − iτ$_{d}$ ( ω + ω $^{′}$) / 2 − τz $^{2}$z ′ 2 τ 3 $_{d}$ωω ′ ] , (7) 3