The model

Suppose identical pseudoatoms (number density $N_0$) which are distributed with certain isotropic medium-range order. We call them simply the atoms. $\overline{Z}$ electrons per atom contribute to the electron gas which forms the ground-state Fermi sphere (Fermi momentum $k_F$). The one-electron states at the Fermi edge are seen as wave packets with one predominating plane wave. However, due to elastic scattering, the planar character is well-developed only within the bounds set by the mean elastic free path, $l_e$. This is typically the extension of the short-scale structures in the medium order range. Thus, the electrons carry a measure of the "planar behaviour" which is confined to volumes of the order $l_e^3$ which are not yet large enough to represent the isotropy of the whole system. As a consequence, scattering processes from one state at the Fermi surface to another one are mediated by atomic arrangements which are likewise planar in ranges of the order $l_e^3$.