BALLISTIC CHARGE-TRANSPORT IN SUPERCONDUCTING WEAK LINKS

Ballistic charge transport through mesoscopic superconductor-normal-metal-superconductor (SNS) microbridges is investigated within the framework of the Keldysh technique. At low voltages we find a peak in the conductance G approximately (l(in)/2a)(1/R0), where lin is the inelastic mean free path, 2a is the normal layer thickness, and R0 is the Sharvin resistance. In this limit the dissipative current is carried by quasiparticles which are trapped in the pair potential well of height DELTA, suffering multiple Andreev reflections and relaxing inside the N layer. For larger voltages eV greater than or similar to (2a/l(in))DELTA quasiparticle are accelerated out of the N layer and relax in superconducting banks. The current-voltage characteristic of the system in this regime is calculated both analytically and numerically. In the intermediate voltage range V approximately DELTA/e we analyze the subharmonic gap structure on the I-V curve and show that this structure is most pronounced in the temperature range T approximately DELTA/k(B) and vanishes for both T --> 0 and T --> T(C). We explain this behavior taking into account the contribution of thermally excited quasiparticles with momenta opposite to the direction of total current flow.