  Transport through a point contact is accurately modelled by assigning to the
junction an ensemble of independent transport channels with possibly different
transmissions. We here argue that for a series of two contacts, coherently
coupled across an island, the transport channels are different from the
ensembles that would describe each contact taken as stand-alone device. We
further show that instead of two sets of channels with manifold cross-links
over the island the double junction can be described by pairs of channels from
both sides coherently coupled together, where each pair, however, has no
coherent connection to the others. This finding will substantially simplify
modelling transport by a Green's functions technique. Additional channels
through only one junction may complete the picture. Finally we discuss how
partial coherence across the island with an appropriate ansatz can be modelled
in the same scheme.
