  Time-reversal invariant superconductors having nodes of vanishing excitation
gap support zero-energy boundary states with topological protection. Existing
expressions for the topological invariant are given in terms of the Hamiltonian
of an infinite system. We give an alternative formulation in terms of the
Andreev reflection matrix of a normal-metal-superconductor interface. This
allows to relate the topological invariant to the angle-resolved Andreev
conductance, also when the boundary state in the superconductor has merged with
the continuum of states in the normal metal. A variety of symmetry classes is
obtained, depending on additional unitary symmetries of the reflection matrix.
We derive conditions for the quantization of the conductance in each symmetry
class and test these on a model for a 2D or 3D superconductor with spin-singlet
and spin-triplet pairing, mixed by Rashba spin-orbit interaction.
