  We report on the observation of the giant photoconductance of a quantum point
contact (QPC) in tunneling regime excited by terahertz radiation. Studied QPCs
are formed in a GaAs/AlGaAs heterostructure with a high-electron-mobility
two-dimensional electron gas. We demonstrate that irradiation of strongly
negatively biased QPCs by laser radiation with frequency f = 0.69 THz and
intensity 50 mW/cm^2 results in two orders of magnitude enhancement of the QPC
conductance. The effect has a superlinear intensity dependence and increases
with the dark conductivity decrease. It is also characterized by strong
polarization and frequency dependencies. We demonstrate that all experimental
findings can be well explained by the photon-mediated tunneling through the
QPC. Corresponding calculations are in a good agreement with the experiment.
