  We show that a spin-up electron from a normal metal entering a superconductor
propagates as a composite object consisting of a spin-down hole and a pair in
the condensate. This leads to a factorization of the non local conductance as
two local Andreev reflections at both interfaces and one propagation in the
superconductor, which is tested numerically within a one dimensional toy model
of reflectionless tunneling. Small area junctions are characterized by non
local conductance fluctuations. A treatment ignoring weak localization leads to
a Thouless energy inverse proportional to the sample size, as observed in the
numerical simulations. We show that weak localization can have a strong effect,
and leads to a coupling between evanescent quasiparticles and the condensate by
Andreev reflections ``internal'' to the superconductor.
