  The Negative Differential Conductivity and Transient Negative Mobility
effects in xenon gas are analyzed by a first-principles particle simulation
technique and via an approximate solution of the Boltzmann transport equation
(BE). The particle simulation method is devoid of the approximations that are
traditionally adopted in the BE solutions in which (i) the distribution
function is searched for in a two- term form, (ii) the Coulomb part of the
collision integral for the anisotropic part of the distribution function is
neglected, (iii) Coulomb collisions are treated as binary events, and (iv) the
range of the electron-electron interaction is limited to a cutoff distance. The
results obtained from the two methods are, for both effects, in good
qualitative agreement, small differences are attributed to the approximations
listed above.
