Cyclotron resonance harmonics in the ac response of a 2D electron gas with smooth disorder

The frequency-dependent conductivity ${\sigma }_{xx}(\omega )$ of 2D electrons subjected to a transverse magnetic field and smooth disorder is calculated. The interplay of Landau quantization and disorder scattering gives rise to an oscillatory structure that survives in the high-temperature limit. The relation to recent experiments on photoconductivity by Zudov {\it et al.} and Mani {\it et al.} is discussed.