Anisotropy of the dispersion relation generates anisotropy in the physical properties of the system. Here the anisotropy parameter is set to A=0.5. This gives a ratio for the effective masses near k=0, m_xx/m_yy = 0.5. The electric field has equal x- and y-components, as may be seen by the orientation of the individual electron trajectories. Running this preset briefly, the graph shows that the average y-velocity is less than the average x-velocity, though in k-space the trajectories have equal x- and y-components. A slight generalization of the Sommerfeld model would imply a ratio of the average or drift velocities equal to the inverse of the effective mass ratio, v_y/v_x = m_xx/m_yy.
Change the Fermi energy to 3.5 eV and run again for 20 or 30 ps. Use the graph to estimate the ratio of the average velocities, v_y/v_x. How can it be less than 0.3 when the effective mass ratio is 0.5?