  A highly superlinear in radiation intensity photoconductance induced by
terahertz laser radiation with moderate intensities has been observed in
quantum point contacts made of GaAs quantum wells operating in the deep
tunneling regime. For very low values of the normalized dark conductance
$G_{\rm dark}/ G_0 \approx 10^{-6}$, with the conductance quantum $G_0=2e^2/h$,
the photoconductance scales exponentially with the radiation intensity, so that
already at $ 100 \text{ mW}/\text{cm}^2$ it increases by almost four orders of
magnitude. This effect is observed for a radiation electric field oriented
along the source drain direction. We provide model considerations of the effect
and attribute it to the variation of the tunneling barrier height by the
radiation field made possible by local diffraction effects. We also demonstrate
that cyclotron resonance due to an external magnetic field manifests itself in
the photoconductance completely suppressing the photoresponse.
