Editing Free electron model (section)

=== Mean free path ===
Notice that without the relaxation time approximation, there is no reason for the electrons to deflect their motion, as there are no interactions, thus the [[mean free path]] should be infinite. The Drude model considered the mean free path of electrons to be close to the distance between ions in the material, implying the earlier conclusion that the [[Diffusion|diffusive motion]] of the electrons was due to collisions with the ions. The mean free paths in the free electron model are instead given by <math display="inline">\lambda=v_{\rm F}\tau</math> (where <math display="inline">v_{\rm F}=\sqrt{2E_{\rm F}/m_e}</math> is the Fermi speed) and are in the order of hundreds of [[ångström]]s, at least one order of magnitude larger than any possible classical calculation.<ref name=":6" group="Ashcroft & Mermin">{{Harvnb|Ashcroft|Mermin|1976|pp=52}}</ref> The mean free path is then not a result of electron–ion collisions but instead is related to imperfections in the material, either due to [[Crystallographic defect|defects]] and impurities in the metal, or due to thermal fluctuations.<ref>{{Cite web|url=https://unlcms.unl.edu/cas/physics/tsymbal/teaching/SSP-927/Section%2008_Electron_Transport.pdf|title=Electronic Transport|last=Tsymbal|first=Evgeny|date=2008|website=University of Nebraska-Lincoln|access-date=2018-04-21}}</ref>