Vector-asymmetry of the quantum point contact conductance

We have investigated a non-linearity of quantum point contacts (QPCs) by measuring their differential resistance, dVdI
, and the rectification, i.e. a dc voltage response Vrect to an applied ac curent. The differential resistance has an almost step-like behavior, i.e. the resistance is constant in a wide range of small currents but depends on current direction. The value of the step is of the order of a few tens of Ohms against a background of several kΩ and the step occurs over a very small bias less than kTe. This current asymmetry causes an almost linear dependence of the rectified voltage on applied ac current amplitude. The step height varies strongly with gate-voltage and is to some extent correlated with the quantized conductance plateaus.