  We explore charge imbalance in mesoscopic normal-metal/superconductor
multiterminal structures at very low temperatures. The investigated samples,
fabricated by e-beam lithography and shadow evaporation, consist of a
superconducting aluminum bar with several copper wires forming tunnel contacts
at different distances from each other. We have measured in detail the local
and non-local conductance of these structures as a function of the applied bias
voltage V, the applied magnetic field B, the temperature T and the contact
distance d. From these data the charge-imbalance relaxation length lambda_Q* is
derived. The bias-resolved measurements show a transition from dominant elastic
scattering close to the energy gap to an inelastic two-stage relaxation at
higher bias. We observe a strong suppression of charge imbalance with magnetic
field, which can be directly linked to the pair-breaking parameter. In
contrast, practically no temperature dependence of the charge-imbalance signal
was observed below 0.5 K. These results are relevant for the investigation of
other non-local effects such as crossed Andreev reflexion and spin diffusion.
