AC JOSEPHSON EFFECT IN A SINGLE QUANTUM CHANNEL

We have calculated all the components of the current in a short one-dimensional channel between two superconductors for arbitrary voltages and transparencies $D$ of the channel. We demonstrate that in the ballistic limit ($D\simeq 1$), the crossover between the quasistationary evolution of the Josephson phase difference $\varphi$ at small voltages and transport by multiple Andreev reflections at larger voltages can be described as the Landau-Zener transition induced by finite reflection in the channel. For perfect transmission and vanishing energy relaxation rate the stationary current-phase relation is never recovered, and $I(\varphi)=I_c \mid \sin \varphi /2 \mid \mbox{sign} V$ for arbitrary small voltages.