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 RN^-1, 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 RN of the order of e².