The Drude model assumed that there is no Pauli exclusion principle, but in Sommerfeld model we have two electron with spin up and down in each energy state. Electrons occupy the states up to the Fermi energy, but only electrons close to the Fermi energy can move and scatter and electrons below the Fermi energy must stay still. That is the consequence of Pauli exclusion principle.
Very much like the shell models of atoms in chemistry, etc. Only the outermost electrons are able to be affected by outside forces while the innermost electrons are shielded from these outside forces by outer electrons.
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