Microwave conductivity resonance of two-dimensional hole system

We have measured microwave conductivity, Re(sigma(xx)), of a high quality two-dimensional hole system (2DHS) in the high magnetic field (B) insulating phase for frequency (f) between 0.2 and 9 GHz. For upsilon < 0.3, Re(sigma(xx)) vs f shows a well-defined peak at frequency f(pk). AS B is increased, the maximal Re(sigma(xx)) increases. The resonance Q (f(pk) divided by full line width at half maximum) increases linearly with B. f(pk) VS B increases, but is nearly flat, at 1.25 GHz, for B greater than or equal to 10 T. Oscillator strength, S, measured by integrating the spectrum, is likewise flat for B greater than or equal to 10 T. The simultaneously B-independent S and f(pk) are inconsistent with the conventional (Fukuyama-Lee) interpretation of the resonance as a Wigner crystal pinning mode.