Ballistic transport in superconducting weak links in a microwave field

Nonequilibrium effects and their impact on charge transport in superconducting ballistic weak links biased by an ac voltage are investigated microscopically. The microwave field destroys the phase coherence during the multiple Andreev reflection cycle and leads to effective cooling of subgap quasiparticles accelerated due to multiple Andreev reflection. For small bias voltages this effect results in a strong supression of both the excess current and the conductance of the weak link. At higher voltages (of the order of the gap) sideband subharmonic gap structures of the I-V curve are found.