  We derive the non-abelian generalization of the Furry approximation which
describes the transverse dynamical evolution of a hard projectile parton inside
a spatially extended colour target field. This provides a unified starting
point for the target rest frame description of the nuclear dependence of a
large class of observables. For the case of the virtual gamma -> q-qbar
photoabsorption cross section, we investigate then in detail under which
conditions the nuclear dependence encoded in the Furry wavefunctions can be
parametrized by a q-qbar QCD dipole cross section. The important condition is
colour triviality, i.e., the property that for arbitrary N-fold rescattering
contributions the only non-vanishing colour trace is N_c C_F^N. We give proofs
for the colour triviality of the inelastic, diffractive and total
photoabsorption cross section measured inclusively or with one jet resolved in
the final state. Also, we list examples for which colour interference effects
remain. Colour triviality allows us to write the gamma -> q qbar contribution
to the DIS nuclear structure function F_2 for small Bjorken x in terms of a
path integral which describes the transverse size evolution of the q-qbar pair
in the nuclear colour field. This expression reduces in an opacity expansion to
the N=1 result of Nikolaev and Zakharov, and in the eikonal approximation to
the Glauber-type rescattering formulas first derived by Mueller. In the
harmonic oscillator approximation of the path integral, we quantify deviations
from the eikonal limit. Their onset is characterized by the scales L/l_f and
E_t L which relate the longitudinal extension L of the nuclear target to the
coherence length l_f and the total transverse energy E_t accumulated by the
q-qbar pair.
