Zero-bias anomaly of quantum point contacts in the low-conductance limit

Most quantum point contacts (QPCs) fabricated in high-mobility two-dimensional electron gases show a zero-bias conductance peak near pinchoff but the origin of this peak remains a mystery. Previous experiments have primarily focused on the zero-bias peak at moderate conductance, in the range 1-2e(2)/h. Here, measurements are presented of zero-bias peaks that persist down to 10(-4)e(2)/h. Magnetic field and temperature dependencies of the zero-bias peak in the low-conductance limit are qualitatively different from the analogous phenomenology at higher conductance with implications for existing theoretical models of transport in lowdensity QPCs.