Ballistic conductance steps and electron focusing in long samples

Abstract

Carefully controlled wet etching of high mobility GaAs-AlGaAs heterostructures has allowed us to make 2 micron long, globally gated quantum wires that exhibit quantized conductance steps. The steps have been observed previously only in very short `point contacts' whose width and length were comparable. The new wires have length to width ratios much greater than 10 near pinch off. The unusual result is attributed to very clean sidewalls that result from the gentle etching process. Loops exhibit quantized steps but with step heights that result from non-trivial addition rules of several contacts. The loops also exhibit a unique classical electron focussing when the cyclotron orbit matches the loop size and Aharonov-Bohm oscillations over a wide range of resistances.