We investigate the dependence of the intensity of radiation due to high-harmonic generation (HHG) as a function of the wavelength λ of a few-cycle driver field. Superimposed on a smooth power-law dependence observed previously, strong and rapid fluctuations on a fine λ scale are observed. The origin of these fluctuations can be identified in terms of quantum path interferences with several orbits significantly contributing. The dependence on the pulse shape and pulse length of the driver is analyzed. We discuss the relation to well-known channel closing effects with emphasis on effects of the ultrashort duration of the pulse.