  We present data on mobility lifetime, $\tau_t$, quantum lifetime, $\tau_q$,
and cyclotron resonance lifetime, $\tau_{CR}$, of a sequence of high-mobility
two-dimensional electron gases in the AlGaN/GaN system, covering a density
range of $\sim1-4.5\times10^{12}$cm$^{-2}$. We observe a large discrepancy
between $\tau_q$ and $\tau_{CR}$ ($\tau_q\sim\tau_{CR}$/6) and explain it as
the result of density fluctuations of only a few percent. Therefore, only
$\tau_{CR}$ --and not $\tau_q$ -- is a reliable measure of the time between
electron scattering events in these specimens. The ratio $\tau_t / \tau_{CR}$
increases with increasing density in this series of samples, but scattering
over this density range remains predominantly in the large-angle scattering
regime.
