Linear and nonlinear conductance of ballistic quantum wires with hybrid confinement

We characterize the linear and nonlinear electron transport in quantum point contacts (QPCs) realized by a hybrid combination of etching and gating. We demonstrate that the strong electron confinement generated through this hybrid QPC process results in quantized subbands with a large energy separation, leading to the observation of robust one-dimensional quantum-transport effects. Measurements of the nonlinear differential conductance reveal an unexpected bunching of curves at 0.20-0.25×2 e2 /h, rather than at the expected value of 0.5×2 e2 /h. Application of a simple analytical model indicates that this bunching is associated with the highly asymmetric manner in which the source-drain voltage is dropped across the QPC near pinch-off and under nonequilibrium conditions.