Transition to Ohmic conduction in ultrasmall tunnel junctions

The effective dc conductance of an ultrasmall tunnel junction is calculated from the functional-integral description beyond the weak-coupling limit by employing the self-consistent harmonic approximation. With increase in the normal conductance $R_N^{-1\mathrm{}}$, the effective conductance exhibits a crossover from the weak-coupling regime, dominated by thermal activation over the Coulomb gap to the Ohmic behavior in the strong-coupling limit. As the temperature and capacitance approach zero, the results indicate a possibility of a precipitous transition for $R_N$ of the order of $\frac{\hslash }{e^2}$.