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Ballistic charge transport through mesoscopic superconductornormal-metalsuperconductor (SNS) microbridges is investigated within the framework of the Keldysh technique. At low voltages we find a peak in the conductance G (l'; /2a)(1/Ro), where I;"is the inelastic mean free path, 2a is the normal layer thickness, and Ro is the Sharvin resistance. In this limit the dissipative current is carried by quasiparticles which are trapped in the pair potential well of height 4, sufFering multiple Andreev refiections and relaxing inside the N layer. For larger voltages eV (2a/I; )6 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 4/e we analyze the subharmonic gap structure on the I Vcurveand show that this structure is most pronounced in the temperature range T 4/kn and vanishes for both T~0 and T~T~. We explain this behavior taking into account the contribution of thermally excited quasiparticles with momenta opposite to the direction of total current Bow.

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Nonequilibrium effects and their impact on a charge transport in superconducting ballistic weak links biased by an ac voltage are investigated within the framework of the Keldysh technique. We demonstrate that the microwave field destroys the phase coherence during the multiple Andreev reflection cycle and leads to the 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. In the opposite limit of large bias voltages the excess current remains unaffected. We also demonstrate that a simple Boltzmann kinetic approach becomes inadequate if an ac voltage bias is applied to the weak link. 74.40.+k,74.50.+r,74.80.Fp Multiple Andreev reflection leads to excitation of quasiparticles in voltage biased superconducting weak links [1]. As a result the quasiparticle distribution function for such systems is driven out of equilibrium already at small voltages. In the case of a time independent voltage bias charge transport in ballistic superconducting weak links in the presence of such nonequilibrium effects has been studied by Octavio et al. [1] within the framework of a simple classical Boltzmann kinetic equation. This so called OTBK model was then widely used in a large number of experimental as well as theoretical works.Although the OTBK model provides a transparent physical picture of multiple Andreev reflection and dissipative charge transport in superconducting weak links it remained unclear if (and/or under which conditions) this model is sufficient to describe quantum nonequilibrium effects in such systems. More recently a rigorous theory of charge transport in ballistic superconductor-normal metal-superconductor (SNS) structures in the presence of a constant voltage bias has been developed by means of the Keldysh technique [2,3]. In the absence of inelastic relaxation the result for a time-averaged dissipative current across the system obtained in [2,3] exactly coincides with that derived from the OTBK model thus providing a formal justification for the OTBK results [1].How far can one go applying the OTBK model to various nonequilibrium effects in superconducting weak links? Is the agreement between the results [1] and [2,3] specific for ballistic weak links biased by a dc voltage or further generalization of the OTBK model (see e.g. [4]) is possible?In this Letter we will study nonequilibrium effects in superconducting microconstrictions in the presence of a timedependent voltage within the framework of the Keldysh technique. We shall determine the distribution function and obtain the current voltage characteristics (CVC) of the weak links. We will demonstrate that photon absorbtion and emission processes in the weak link (in combination with multiple Andreev reflection) may have a strong impact on the transport properties of the system leading to effective heating or cooling of subgap quasiparticles and to a strong suppression of the current in t...