where k F stands for the Fermi wave number that depends on the electron density and is the absolute value of the wave number of the highest occupied electron state. Thus, because the Sommerfeld parameter γ γ is inversely proportional to ϵ ϵ F , γ γ is proportional to the particle's mass and for high values of γ γ , the metal behaves as a Fermi gas in which the conduction electrons have a high thermal effective mass.