Quantum particle in a washboard potential. II. Nonlinear mobility and the Josephson junction

We obtain an analytic expression for the nonlinear mobility of a quantum particle moving down a cosine potential in the presence of a finite bias force in the small viscosity limit. This is used to explain some experimental observations in Josephson junctions in which the Coulomb blockade is important. The result is based on a resummation of a series expansion of the mobility in V₀ (the strength of the cosine potential) and is applicable to a variety of regimes in the parameter space.