  The amplitude and phase of Shubnikov-de Haas oscillations have been analyzed
in detail for two-dimensional electron gases subjected to a weak unidirectional
periodic potential modulation. The amplitude is suppressed, accompanied by
inversion of the phase, at the maximum bandwidth conditions at low magnetic
fields. The suppression is gradually taken over by the enhancement with the
increase of the magnetic fields. The suppression and the enhancement are
attributable to the collisional and the diffusion contribution of the modulated
potential to the conductivity, respectively, the former (the latter) being
dominant at low (high) magnetic fields. A theoretical calculation that takes
the two types of contributions into account shows semi-quantitative agreement
with experimental traces.
