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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 

