  We measure the longitudinal conductivity $\sigma_{xx}$ at frequencies $1.246
{\rm GHz} \le f \le 10.05$ GHz over a range of temperatures $235 {\rm mK} \le T
\le 4.2$ K with particular emphasis on the Quantum Hall plateaus. We find that
$Re(\sigma_{xx})$ scales linearly with frequency for a range of magnetic field
around the center of the plateaus, i.e. where $\sigma_{xx}(\omega) \gg
\sigma_{xx}^{DC}$. The width of this scaling region decreases with higher
temperature and vanishes by 1.2 K altogether. Comparison between localization
length determined from $\sigma_{xx}(\omega)$ and DC measurements on the same
wafer show good agreement.
