Crossover from weak to strong coupling in tunneling has been studied with adjustable microscopic tunnel junctions consisting of platinum electrodes separated by a helium film. The depth and width of the zero-bias Coulomb-blockade feature decrease with increasing conductance, disappearing at ≊1.4${\mathit{e}}^2$/h. This behavior is interpreted using a model in which tunneling between electrode ‘‘asperities’’ effectively occurs in a single quantum channel.

• Received 10 June 1991

DOI:https://doi.org/10.1103/PhysRevB.44.12868