  Using simultaneous measurements of the attenuation and velocity of surface
acoustic waves propagating along GaAs/Al$_{0.3}$Ga$_{0.7}$As heterostructures,
complex ac conductance of the latters has been determined. In the magnetic
fields corresponding to the middles of the Hall plateaus both the ac
conductance, $\sigma (\omega)$, and the sheet electron density, $n_s$, in the
two-dimensional conducting layer turn out to be dependent on the samples'
cooling rate. As a result, the sample ``remembers'' the cooling conditions. The
complex conductance is strongly dependent on an infrared illumination which
also changes both $\sigma (\omega)$ and $n_s$. Remarkably, the correlation
between $\sigma (\omega)$ and $n_s$ is \emph{universal}, i.e. it is independent
of the way to change these quantities. The results are attributed to
two-electron defects (so-called $DX^-$ centers) located in the Si doped layer.
