  We have measured the complex conductivity $\sigma_{xx}$ of a two-dimensional
electron system in the quantum Hall regime up to frequencies of 6 GHz at
electron temperatures below 100 mK. Using both its imaginary and real part we
show that $\sigma_{xx}$ can be scaled to a single function for different
frequencies and for all investigated transitions between plateaus in the
quantum Hall effect. Additionally, the conductivity in the variable-range
hopping regime is used for a direct evaluation of the localization length
$\xi$. Even for large filing factor distances $\delta \nu$ from the critical
point we find $\xi \propto \delta \nu^{-\gamma}$ with a scaling exponent
$\gamma=2.3$.
