  The chiral induced spin selectivity (CISS) in layers of helical molecules
gained considerable attention in the emerging field of spintronics, because the
effect enables spin-filter devices under ambient conditions. Several
theoretical studies have been carried out to explain this effect on a
microscopic scale, but the origin of the effect is still controversial. In
particular the role of spin-flip scattering during electron transport is an
open issue. In this study we describe the electron and spin transport by rate
equations including spin-dependent losses and spin-flip scattering. We reduce
the problem to the solution of the Riccati differential equation to obtain
analytical solutions. The results allow to determine and interpret the strength
and scalability of CISS based spin-filters from experimental data or quantum
mechanical models. For the helical systems studied experimentally so far it
turns out that spin-flip scattering plays a minor role.
