Ballistic electronic conductance of a wide orifice

For the ballistic motion of a two-dimensional noninteracting electron gas we compute the quantum-mechanical conductance of a rectangle of length L whose lateral dimension a is greater than that of the incoming and outgoing wires, b. The global structure of the conductance when the lateral dimension increases reveals slow decay, contrary to what one expects from the simple law of addition of resistances in parallel. Closer inspection shows an oscillatory behavior with well-defined peaks at kFa/n. No quantized plateaus (which occur for a<b) exist. A simple interpretation is given in terms of localization and resonance tunneling.