Ionization by Impact Electrons in Solids: Electron Mean Free Path Fitted   Over a Wide Energy Range
  We propose a simple formula for fitting the electron mean free paths in solids both at high and at low electron energies. The free-electron-gas approximation used for predicting electron mean free paths is no longer valid at low energies (E < 50 eV), as the band structure effects become significant at those energies. Therefore we include the results of the band structure calculations in our fit. Finally, we apply the fit to 9 elements and 2 compounds. 
B. Ziaja, R.A. London, J. Hajdu