when calculating the frequencies of 1\H's oscillation using De Haas-van Alphen, we need to find the extremal cross-section of the fermi surface perpendicular to the magnetic field direction. in 3d i can understand this. but when talking about a 2d lattice where the magnetic field is perpendicular to the lattice plane, what do i take as the extremal fermi surface cross section? and another question, back to 3D - if i have several bands, each of them have different fermi surface, right? so i need to take the extremal cross section at each of them?