DISSIPATIVE CHARGE-TRANSPORT IN MESOSCOPIC SNS JUNCTIONS

Ballistic charge transport through mesoscopic superconductor-norma met al-superconductor (SNS) microbridges is investigated on the basis of t he Keldysh technique. At low voltages we find a peak in the conductanc e which is caused by multiple Andreev reflection of quasiparticles tra pped in the pair potential well of the N-layer in the presence of inel astic relaxation there. For larger voltages inelastic relaxation in th e N-layer plays no role, quasiparticles are accelerated out of the N-l ayer and relax in superconducting banks. We demonstrate that even at e V T << Delta the system has a finite subgap conductance, i.e. dissipat ive current can flow through a superconducting constriction both with and without normal metal inclusion. In this case dissipation arises fr om pairbreaking due to acceleration of virtually created subgap quasip articles by the electric field. In the voltage range V similar to Delt a/e we analyze the temperature dependence of the subharmonic gap struc ture which turns out to be most pronounced at T similar to Delta and v anishes at T --> 0 and T --> T-c. This behavior is the result of a cou nter current induced by thermally excited quasiparticles.