We calculate the g factor at every point of the Fermi surface of aluminium by using a classical \'four orthogonalised plane waves scheme and by introducing the spinorbit potential as a perturbation. An important difficulty remains, linked to the choice of the wavefunction phase. Moreover we propose a phenomenological model based on the narrowing of the g distribution by two types of motion: a random one corresponding to the diffusion of electrons on the crystalline imperfections and a coherent one around the cyclotron orbits. A qualitative model accounts relatilely well for the spin resonance experimental data
