  In two previous papers we studied the problem of electronic properties in a
system with long-ranged helimagnetic order caused by itinerant electrons. A
standard many-fermion formalism was used. The calculations were quite tedious
because different spin projections were coupled in the action, and because of
the inhomogeneous nature of a system with long-ranged helimagnetic order. Here
we introduce a canonical transformation that diagonalizes the action in spin
space, and maps the problem onto a homogeneous fermion problem. This
transformation to quasiparticle degrees of freedom greatly simplifies the
calculations. We use the quasiparticle action to calculate single-particle
properties, in particular the single-particle relaxation rate. We first
reproduce our previous results for clean systems in a simpler fashion, and then
study the much more complicated problem of three-dimensional itinerant
helimagnets in the presence of an elastic relaxation rate 1/tau due to
nonmagnetic quenched disorder. Our most important result involves the
temperature dependence of the single-particle relaxation rate in the ballistic
limit, tau^2 T\epsilon_F > 1, for which we find a linear temperature
dependence. We show how this result is related to a similar result found in
nonmagnetic two-dimensional disordered metals.
