“…For a complete picture of the solution as a function of t and p, we have to solve the equation for several values of q. With this method, the equation of motion of the Wigner function can be brought to the simple forṁ w =Mw, (17) where the tilde notation indicates as before that the corresponding functions are evaluated along the path p = π q (t), and the resulting function is then understood as a function of q, e. g.,w(t, q) ≡ w(t, p = π q (t)). For a suitable choice of π 0 , the characteristics π q (t) coincide with the classical momentum space paths of electrons in the external field according to…”