  Dynamical magnetic impurities (MI) are considered as a possible origin for
suppression of the ballistic helical transport on edges of 2D topological
insulators. The MIs provide a spin-flip backscattering of itinerant helical
electrons. Such a backscattering reduces the ballistic conductance if the
exchange interaction between the MI and the electrons is anisotropic and the
Kondo screening is unimportant. It is well-known that the isotropic MIs do not
suppress the helical transport in systems with axial spin symmetry of the
electrons. We show that, if this symmetry is broken, the isotropic MI acquires
an effective anisotropy and suppresses the helical conductance. The peculiar
underlying mechanism is a successive backscattering of the electrons which
propagate in the same direction and have different energies. The respective
correction to the linear conductance is determined by the allowed phase space
of the electrons and scales with temperature as T^4. Hence, it disappears at
small temperatures. This qualitatively distinguishes effects governed by the
MIs with the induced and bare anisotropy; the latter is temperature
independent. If T is smaller than the applied bias, finite e V, the allowed
phase space is provided by the bias and the differential conductance scales as
(e V)^4.
