Admittance of a quantum point contact

The high frequency conductance G of a quantum point contact was studied for f <= 300 MHz. While the real part 91(G) remains unchanged by frequency we observe a stepwise change of the imaginary part f(G) when adding additional conductance channels. The step height Delta f(G) is linear in frequency with a positive slope and thus has capacitive character with a Delta C per channel of a few fF. (C) 2007 Elsevier B.V. All rights reserved.