  Dissipative electromagnetic response and scattering geometry are potential
sources for the appearance of a negative Casimir entropy. We show that the
dissipative contribution familiar from the plane-plane geometry appears also in
the plane-sphere and the sphere-sphere geometries and adds to the negative
Casimir entropy known to exist in these geometries even for perfectly
reflecting objects. Taking the sphere-sphere geometry as an example, we carry
out a scattering-channel analysis which allows to distinguish between the
contributions of different polarizations. We demonstrate that dissipation and
geometry share a common feature making possible negative values of the Casimir
entropy. In both cases there exists a scattering channel whose contribution to
the Casimir free energy vanishes in the high-temperature limit. While the
mode-mixing channel is associated with the geometric origin, the transverse
electric channel is associated with the dissipative origin of the negative
Casimir entropy. By going beyond the Rayleigh limit, we find even for large
distances that negative Casimir entropies can occur also for Drude-type metals
provided the dissipation strength is sufficiently small.
