Is the free-electron model synonymous with Drude model? or is Drude model a special subset of the free-electron model? I have seen texts that refer to a "free-electron model" and do not mention Drude's name. and i have also seen texts use these two terms interchangeably. Sorry, i have no specific examples. I'm just looking for a generalization. Thanks guys. also as a small side question: whenever a text refers to energy bands, fermi levels, or potential wells, is this necessarily a "quantum approach"? or is this why the Drude model is considered "semi-classical"?