  Using acoustic methods the complex high-frequency conductance of
high-mobility $n$-GaAs/AlGaAs heterostructures was determined in magnetic
fields 12$\div$18~T. Based on the observed frequency and temperature
dependences we conclude that in the investigated magnetic field range and at
sufficiently low temperatures, $T \lesssim 200$~mK, the electron system forms a
Wigner crystal deformed due to pinning by disorder. At some temperature, which
depends on the electron filling factor, the temperature dependences of both
components of the complex conductance get substantially changed. We have
ascribed this rapid change of the conduction mechanism to melting of the Wigner
crystal and study the dependence of the so-defined melting temperature on the
electron filling factor.
