  The nonohmic conductivity of 2D hole gas (2DHG) in single
$GaAsIn_{0.2}Ga_{0.8}AsGaAs$ quantum well structures within the temperature
range of 1.4 - 4.2K, the carrier's densities $p=(1.5-8)\cdot10^{15}m^{-2}$ and
a wide range of conductivities $(10^{-4}-100)G_0$ ($G_0=e^2/\pi\,h$) was
investigated. It was shown that at conductivity $\sigma>G_0$ the energy
relaxation rate $P(T_h,T_L)$ is well described by the conventional theory (P.J.
Price J. Appl. Phys. 53, 6863 (1982)), which takes into account scattering on
acoustic phonons with both piezoelectric and deformational potential coupling
to holes. At the conductivity range $0.01G_0<\sigma<G_0$ energy the relaxation
rate significantly deviates down from the theoretical value. The analysis of
$\frac{dP}{d\sigma}$ at different lattice temperature $T_L$ shows that this
deviation does not result from crossover to the hopping conductivity, which
occurs at $\sigma<10^{-2}$, but from the Pippard ineffectiveness.
