ac response of quantum point contacts with a split-gate configuration

The alternating-current response of a quantum point contact (QPC) is numerically investigated using the nonequilibrium Green function method combined with an effective mass theory. We found that the susceptance of a QPC increases stepwise with increasing gate voltage, when the width of the quantization plateau in the gate voltage-conductance curve is narrower than the width of the region where the conductance changes gradually. We also show that the height of a susceptance step is proportional to the ac-bias frequency. These simulation results are in excellent qualitative agreement with recent experimental results. Moreover, we found that the transition from capacitive susceptance to inductive susceptance occurs with increasing gate voltage. The capacitive-inductive transition point is independent of the ac-bias frequency, but it does depend on the contact width