By measuring the ac impedance, we measure for the first time the crossover from diffusive (omegatau < 1) to ballistic (omegatau > 1) transport as a function of frequency (dc to 5 GHz) in a dc contacted 2d electron gas in the low electric field limit, where tau is the momentum scattering time. The geometry is an "ungated HEMT", meaning current flows through an ohmic contact into a 2d electron gas, laterally through the 2d electron gas, and out into a second ohmic contact; the 2d electron gas itself is not gated. At the measurement temperature (4.2 K), the low field mobility is 3.2 x 10(6) cm(2)/V s. We also measure for the first time the frequency dependent contact impedance in this ballistic limit. (C) 2004 Elsevier Ltd. All rights reserved.