The electron velocity distribution function (EVDF) of electron swarms in N2 has a constricted part at an electron energy of around 2-3 eV. This feature has been explained by considering the barrier effect of vibrational excitation collisions that prevents the gain of energy by slow electrons. In order to investigate the barrier effect in detail further, we simulated the electron behaviour around the energy range of the barrier. We found that momentum transfer collisions also contribute to the barrier effect in collaboration with vibrational excitation. In addition, an evaluation of the electron flow in velocity space is thought to indicate a factor by which the presence of the barrier effect in N2 is revealed in the EVDF; the depression of the electron interchange across the barrier is not only due to the barrier effect itself but also due to the small electron transfer associated with the energy loss by electronic excitation.