  We report on spin transport along double-helical molecular systems by
considering various contact configurations and asymmetries between the two
helical strands in the regime of completely coherent charge transport. Our
results reveal that no spin polarization appears in two-terminal molecular
devices when coupled to one-dimensional electrodes. The same holds in the case
of finite-width electrodes if there is a \emph{bottleneck} of one single site
in the system electrode--molecule--electrode. Then, additional dephasing is
necessary to induce spin-filtering effects. In contrast, nonzero spin
polarization is found in molecular devices with multiple terminals or with two
finite-width electrodes, each of them connected to more than one site of the
molecule. Then, the magnitude of spin polarization can be enhanced by
increasing the asymmetry between the two strands. We point out that the
spin-filtering effects could emerge in double-helical molecular devices at low
temperature without dephasing by a proper choice of the electrode number and
the connection between the molecule and the electrodes.
