Coulomb blockade and the intrinsically one-dimensional character of microscopic tunnel junctions

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 {}1.4e²/h. This behavior is interpreted using a model in which tunneling between electrode ``asperities'' effectively occurs in a single quantum channel.