  Based on the fundamental momentum-transfer theorem [Phys. Rev. Lett. 15, 11
(1965)] a novel contribution to the retarding force of metallic systems for
slow intruders is derived. This contribution is associated with sudden
charge-changing cycles during the path of projectiles. The sum of the novel and
the well-known conventional contributions, both expressed in terms of
scattering phase shifts, are used to discuss experimental data obtained for
different targets. It is found that our two-channel modeling, with two
nonlinear channels, improves the agreement between several data and theory and
thus, as predictive modeling, can contribute to the desired convergence between
experimental and theoretical attempts on the retarding force.
