  Two-dimensional (2D) electrons in high-quality nanostructures at low
temperatures can form a viscous fluid. We develop a theory of high-frequency
magnetotransport in such fluid. The time dispersion of viscosity should be
taken into account at the frequencies about and above the rate of
electron-electron collisions. We show that the shear viscosity coefficients as
functions of magnetic field and frequency have the only resonance at the
frequency equal to the doubled cyclotron frequency. We demonstrate that such
resonance manifests itself in the plasmon damping. Apparently, the predicted
resonance is also responsible for the peaks and features in photoresistance and
photovoltage, recently observed on the best-quality GaAs quantum wells. The
last fact should considered as an important evidence of forming a viscous
electron fluid in such structures.
