  To describe momentum isotropization of gluon matter produced in
ultrarelativistic heavy-ion collisions, the transport rate of gluon drift and
the transport collision rates of elastic ($gg \leftrightarrow gg$) as well as
inelastic ($gg \leftrightarrow ggg$) perturbative quantum chromodynamics-
(pQCD) scattering processes are introduced and calculated within the kinetic
parton cascade Boltzmann approach of multiparton scatterings (BAMPS), which
simulates the space-time evolution of partons. We define isotropization as the
development of an anisotropic system as it reaches isotropy. The inverse of the
introduced total transport rate gives the correct time scale of the momentum
isotropization. The contributions of the various scattering processes to the
momentum isotropization can be separated into the transport collision rates. In
contrast to the transport cross section, the transport collision rate has an
indirect but correctly implemented relationship with the collision-angle
distribution. Based on the calculated transport collision rates from BAMPS for
central Au+Au collisions at Relativistic Heavy Ion Collider energies, we show
that pQCD $gg \leftrightarrow ggg$ bremsstrahlung processes isotropize the
momentum five times more efficiently than elastic scatterings. The large
efficiency of the bremsstrahlung stems mainly from its large momentum
deflection. Due to kinematics, $2\to N$ $(N>2)$ production processes allow more
particles to become isotropic in momentum space and thus kinetically
equilibrate more quickly than their back reactions or elastic scatterings. We
also show that the relaxation time in the relaxation time approximation, which
is often used, is strongly momentum dependent and thus cannot serve as a global
quantity that describes kinetic equilibration.
