FIG. 1: Sketch of the principal setup mounted to the mixing chamber. The illustration is not to scale. A copper plate supporting the copper-powder filters is mounted directly to the mixing chamber. An incised copper tube is used to ensure that the STM body, more precisely the sample position, is located in the homogeneous part of the magnetic field generated by a superconducting solenoid. mixing chamber surrounds the STM body. It has been verified that a temperature gradient of less than 20 mK occurs between mixing chamber and STM body. As a test we recorded tunnel spectra of superconducting aluminum ( T$_{c}$ = 1 . 1 K) which exhibited a temperature dependence down to a STM body temperature of ≈ 100 mK. We observed a temperature dependent dI/dV curve down to 200 mK, i.e., T/T$_{c}$ = 0 . 18. The fact that we did not ob- serve a further sharpness of the BCS singularity at lower temperature is probably due to electronic noise. Thus we can conclude in this particular experiment (STM body tempera- ture ≈ 100 mK) that the sample temperature is at least 200 mK. All temperatures given in this article correspond to the STM body temperature determined in the experiment from 4