Non-Boltzmann classical correction to the velocity auto-correlation   function for isotropic scattering in two dimensions
  The classical correction to the velocity auto-correlation function of non-interacting particles due to memory effects, which are beyond the Boltzmann equation, is calculated both analytically and numerically for the case of isotropic scattering in two dimensions. 
Alexander Dmitriev, Michel Dyakonov, Remi Jullien