  We calculate the magnetoconductivity tensor of a 2D electron gas in a 1D
periodic potential and quantising magnetic fields. We study the internal
structure of the Shubnikov-de Haas peaks and analyse recent experimental
results. The electron-electron interaction is accounted for within a screened
Hartree-Fock approximation (HFA), which describes both compressible strips and
an exchange-enhanced spin splitting, unlike the Hartree and the standard HFA.
