  We use random-matrix theory and supersymmetry techniques to work out the
two-point correlation function between states in a hierarchical model which
employs Feshbach's chaining hypothesis: Classes of many-body states are
introduced. Only states within the same or neighboring classes are coupled. We
assume that the density of states per class grows monotonically with class
index. The problem is mapped onto a one-dimensional non-linear sigma model. In
the limit of a large number of states in each class we derive the critical
exponent for the growth of the level density with class index for which
delocalization sets in. From a realistic modelling of the class-dependence of
the level density, we conclude that the model does not predict Fock-space
localization in nuclei.
