Loss of Andreev backscattering in superconducting quantum point contacts

We study effects of magnetic field on the quasiparticle energy spectrum in a superconducting quantum point contact. The supercurrent induced by the magnetic field leads to intermode transitions between the electron waves that pass and do not pass through the constriction. The latter experience normal reflections which couple the states with opposite momenta inside the quantum channel and create a minigap in the low-energy spectrum that depends on the magnetic field.