Broadband microwave spectroscopy of a GaAs point contact

We use a novel broadband microwave spectroscopy measurement to study the source–drain current characteristics of a GaAs quantum point contact (QPC) within the tunnelling regime. The experimental setup was based on the Corbino approach with the notable difference that it was non-invasive to the sample and swept a frequency range between 1 and 10 GHz at low temperatures. Highly aperiodic but completely reproducible conductance fluctuations were observed close to pinch-off as well as regions of negative conductance. We account these fluctuations to be due to quantum interference of ballistic electron trajectories, trapped within the QPC, being modified by the radiation source and influencing the tunnelling rates through a complex potential well structure.