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 (D1) the crossover between the quasistationary evolution of the Josephson phase difference 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 rates the stationary current-phase relation is never recovered, and I sin sgnV for arbitrary small voltages.