For scattering from acoustic phonons, for temperatures well above Debye temperature, the estimated cross section Σ ph is determined from the square of the average vibrational amplitude of a phonon to be proportional to T. The scattering from charged defects (ionized donors or acceptors) leads to the cross section Σ Σ def ∝ ∝ ⟨ v ⟩ − − 4 . This formula is the scattering cross section for "Rutherford scattering", where a point charge (carrier) moves past another point charge (defect) experiencing Coulomb interaction.