ac ballistic transport in a two-dimensional electron gas measured in GaAs/AlGaAs heterostructures

We measure the crossover from diffusive to ballistic transport as a function of frequency in dc contacted high-mobility two-dimensional electron gas structures in GaAs/AlGaAs heterostructures at GHz frequencies. By systematically measuring samples of varying mobility we demonstrate that the crossover frequency scales with mobility as predicted by the Drude model. We find the ohmic contact impedance to be real and independent of frequency for samples with mobility higher than 500 000 cm(2)/V s, while there is a significant capacitive component in parallel with the contact resistance for lower mobility samples.