g ij,ω = g$_{ij,ω}$ − ∑ kl g$_{il,ω}$g$_{kj,ω}$/ ( ∑ kl g$_{kl,ω}$ − iωC ) , g$_{ij,ω}$ = 2 πν$_{s}$ ∫ dε tr [ 1 1$_{i}$ 1 1$_{j}$ − 1 1$_{j}$ S $^{†}$( ε ) 1 1$_{i}$ S ( ε + ω/ 2 π )] × ( f ( ε ) − f ( ε + ω/ 2 π )) /ω, i, j, k, l = 1 , M, (6) Var R$_{4t}$ = ( h 2 e $^{2}$tr Γ ) 2 ∑ i =1 , 2 v =3 4 tr Γ 2 i tr $^{2}$Γ$_{i}$ tr Γ 2 v tr $^{2}$Γ$_{v}$ ∫$_{ωω}$ , (3) Var R$_{2t}$ = 2 β ( h 2 e 2 N$_{1}$ + N$_{2}$ N$_{1}$N$_{2}$N ) 2 1 − 2 iωτ$_{RC}$ (1 iωτ$_{RC}$ ) 2 ∫$_{ωω}$ , (4) ∫$_{ω}$$_{Ω}$ ≡ ∫ + ∞ 0 ( 2 πT sin ωτ/ 2 ℏ ω sinh πT τ/ ℏ ) 2 e τ ( i Ω − 1 /τ d $^{)}$dτ (1 − i Ω τ$_{d}$ ) τ$_{d}$ . (5) 〈 R$_{4t}$ 〉 = 0 , (1) 〈 R$_{2t}$ 〉 = h 2 e 2 tr Γ$_{1}$ + tr Γ$_{2}$ tr Γ$_{1}$tr Γ$_{2}$ [ 1 + δ$_{β}$$_{1}$ tr Γ(1 − iωτ$_{d}$ ) ] . (2) We calculate the universal distribution of the phase of transmitted voltage in a four-terminal ac experiment. wrong: R$_{4t}$ does not have weak localization, and its fluc- tuations are insensitive to magnetic field. The differences are even greater when the dynamics of resistance fluctu- ations is compared: unlike R$_{2t}$ governed both by τ$_{d}$ and τ$_{RC}$ , R$_{4t}$ turns out to depend only on τ$_{d}$ . Surprisingly, fluctuations of R$_{4t}$ are much simpler than UCF, and a sin- gle formula covers the regimes (i-iii) for arbitrary floating probes, imperfections in contacts, and frequency ω . The calculation. We consider a chaotic quantum dot in a multi-probe setup with M ≥ 4 contacts and a gate with capacitance C , see Fig. 1 (a). Each contact j = 1 , ..., M leading to a reservoir with a voltage V$_{jω}$ at the fre- quency ω is characterized by a set of transmission values Γ$_{j}$ of its (imperfect) channels. This diffusive/ballistic dot is in the universal regime, when direct trajectories are absent, and chaos validates random matrix theory (RMT) [16, 17]. The Thouless energy of a closed dot is much larger than the temperature T , mean level spac- ing Δ = 2 π ℏ $^{2}$/ ( m Area), the excitation energy ℏ ω in the ac experiment, and escape rate tr Γ · Δ / 2 π . The total coupling to environment is good, tr Γ ≫ 1, and it speci- fies the electronic dwell-time τ$_{d}$ = h/ (tr ΓΔ) in the dot. The leading Coulomb interaction effect is accounted for by a uniform self-consistent potential of the dot, found from charge conservation [21, 25]. The terms leading to Coulomb blockade are unimportant due to 1 / tr Γ ≪ 1. We use RMT and diagrammatic technique with a small parameter 1 / tr Γ ≪ 1 for an energy-dependent scattering matrix S ( ε ) of a dot with imperfect contacts [20, 26, 27] to find transport statistics. Further we take h = e = 1 and consider spin-degenerate electrons, ν$_{s}$ = 2, the role of spin-orbit in R$_{4t}$ being discussed elsewhere [28]. Universal resistance fluctuations. This Letter com- pares statistics of two- and four-terminal resistances in quantum dots with non-ideal coupling to reservoirs and probes, biased by ac voltages, see Fig. 1(a). To be spe- cific let the current I flow only through contacts 1,2 and measure the voltage drop between 3,4 (or 1,2) to find R$_{4t}$ = V$_{34}$/I (or R$_{2t}$ = V$_{12}$/I ). Each contact j with N$_{j}$ orbital channels is specified by a diagonal matrix Γ$_{j}$ of its channel transmissions, and we assume good dimen- sionless conductances of the probes 1–4, tr Γ$_{j}$ ≫ 1. At arbitrary temperature T and measurement frequency ω , in first two orders in the total QPC conductance tr Γ, the ensemble averaged resistances are given by Linear transport coefficients are found from the screened frequency-dependent degenerate ( M +1) × ( M + 1) conductance matrix g s ij,ω = ∂I$_{iω}$/∂V$_{jω}$ , where Vanishing 〈 R$_{4t}$ 〉 is not sensitive to the presence of time- reversal symmetry (TRS), β = 1, or its breaking by mag- netic field, β = 2. In contrast, 〈 R$_{2t}$ 〉 is corrected by WL depending only on τ$_{d}$ . Similarly, TRS does (not) affect fluctuations of R$_{2t}$ ( R$_{4t}$ ) around their averages: Fluctuations of R$_{4t}$ , Eq. (3), the main result of this paper, are valid for any couplings of floating probes, and imper- fections in the contacts, while Eq. (4) is derived only for ballistic QPCs in a multi-terminal dot. In an effectively zero-dimensional dot universal Eqs. (1–4) do not depend on disorder and contact positions. It is interesting that for conductance there are no re- sults of comparable generality for WL and UCF . We conclude that both weak-localization corrections to the two-terminal resistance, Eq. (2), and fluctuations of the four-terminal one, Eq. (3), are completely insensitive at all frequencies to the RC-time and thus to Coulomb inter- actions. We next explain how these results are obtained, and discuss the dc fluctuations of resistances. Later we generalize our results onto an ac setup and discuss the role of the chosen measurement scheme in fluctuations. DC setup, ω = 0 . First, let us consider dc transport and ballistic QPCs at T = 0, when any tr Γ n j = N$_{j}$ in Eqs. (2,3) and the integral, Eq. (5), is reduced to 1. In this limit the measurements in a two-terminal dot are not affected by the Coulomb interaction, and both 〈 R$_{2t}$ 〉 and its fluctuations are diminished by the TRS-breaking [29]. In a multi-terminal setup, however, chaotic scattering re- sults in a random voltage drop between any pair of volt- age probes. As a consequence, the average R$_{4t}$ vanishes, 〈 R$_{4t}$ 〉 = 0, and does not have a WL contribution. In- deed, R$_{2t}$ = ( g$_{33}$g$_{44}$ − g$_{34}$g$_{43}$ ) · det R is symmetric with respect to 1 ↔ 2, and its WL is the first correction due to self-intersections of an electronic trajectory from 1 to 2. On the other hand, R$_{4t}$ = ( g$_{31}$g$_{42}$ − g$_{32}$g$_{41}$ ) · det R [4] 2