  We studied ballistic transport across a quantum point contact (QPC) defined
in a high-quality, GaAs (311)A two-dimensional (2D) hole system using shallow
etching and top-gating. The QPC conductance exhibits up to 11 quantized
plateaus. The ballistic one-dimensional subbands are tuned by changing the
lateral confinement and the Fermi energy of the holes in the QPC. We
demonstrate that the positions of the plateaus (in gate-voltage), the
source-drain data, and the negative magneto-resistance data can be understood
in a simple model that takes into account the variation, with gate bias, of the
hole density and the width of the QPC conducting channel.
