  A counter-intuitive disappearance of the giant terahertz photoconductance of
a quantum point contact (QPC) under increase in the photon energy, which was
discovered experimentally (Otteneder et al., Phys. Rev. Applied 10 (2018)
014015) and studied by the numerical calculations of the photon-stimulated
transport (O.A. Tkachenko et al., JETP Lett. 108 (2018) 396), is explained here
by using qualitative considerations about the momentum conservation upon
absorption of terahertz photons. The spectra of photon-stimulated transmission
through a smooth one-dimensional barrier are calculated on the basis of the
perturbation theory. These calculations also predict the spectral maxima for
optical transitions from the Fermi level to the top of the potential barrier.
Within the proposed physical picture, the widths of the spectral maxima are
estimated, and the evolution of the shape of the spectra with a change in the
position of the Fermi level is qualitatively explained.
