Coherent low-energy charge transport in a diffusive S-N-S junction.

We have studied the current-voltage characteristics of diffusive mesoscopic Nb-Cu-Nb Josephson junctions with highly transparent Nb-Cu interfaces. We consider the low-voltage and high-temperature regime eV < epsilon(c) < k(B)T, where epsilon(c) is the Thouless energy. The observed excess current as well as the observed subharmonic Shapiro steps under microwave irradiation suggest the occurrence of low-energy coherent multiple Andreev reflection.