  We investigate the connection of recently identified fine-scale oscillations
in the dependence of the yield of the high-harmonic generation (HHG) on
wavelength lambda of a few-cycle laser pulse [K. Schiessl, K.L. Ishikawa, E.
Persson, and J. Burgdorfer, Phys. Rev. Lett. 99, 253903 (2007)] to the
well-known channel closing (CC) effect. Using the Lewenstein model of HHG, we
identify the origin of the oscillations as quantum interference of many
rescattering trajectories. By studying the simultaneous variations with
intensity and wavelength, different models for the interference of channel
closing peaks can be tested. Contrary to theoretical predictions for
short-range potentials, the peaks are not located neither at nor just below the
CC condition, but a significant shift is observed. The long Coulomb tail of the
atomic potential is identified as the origin of the shift.
