  We present high frequency measurements of the diagonal conductivity sigma_xx
of a two dimensional electron system in the integer quantum Hall regime. The
width of the sigma_xx peaks between QHE minima is analyzed within the framework
of scaling theory using both temperature T=100-700 mK and frequency f <= 6 GHz
in a two parameter scaling ansatz. For the plateau transition width we find
scaling behaviour for both its temperature dependence as well as its frequency
dependence. However, the corresponding scaling exponent for temperature
kappa=0.42 significantly differs from the one deduced for frequency scaling
(c=0.6). Additionally we use the high frequency experiments to suppress the
contact resistances that strongly influences DC measurements. We find an
intrinsic critical conductivity sigma_c=0.17e^2/h, virtually independent of
temperature and filling factor, and deviating significantly from the proposed
universal value 0.5e^2/h.
