Numerically calculated running momentum diffusion coefficient for the cases of R_A = 10^-4 (solid line) and R_A = 10^-3 (dashed line). We note that for the last part of the lower curve, only 10³ particles could be used, while the rest was obtained from a simulation with 10⁴ particles. The two diamonds illustrate the quasi-linear result from Eq. (B.2)in Appendix B.