  We discuss transport through interferometer formed by helical edge states
tunnel-coupled to metallic leads. We focus on the experimentally relevant case
of relatively high temperature as compared to the level spacing and discuss a
response of the setup to the external magnetic flux $\phi$ (measured in units
of flux quantum) piercing the area encompassed by edge states. We demonstrate
that tunneling conductance of the interferometer is structureless in ballistic
case but shows a sharp antiresonances, as a function of magnetic flux $\phi$ -
with the period 1/2 - in the presence of magnetic impurity. We interpret the
resonance behavior as a coherent enhancement of backward scattering off
magnetic impurity at integer and half-integer values of flux, which is
accompanied by suppression of the effective scattering at other values of flux.
Both enhancement and suppression are due to the interference of processes with
multiple returns to magnetic impurity after a number of clockwise and
counterclockwise revolutions around setup. This phenomenon is similar to the
well-known weak-localization-induced enhancement of backscattering in
disordered systems. The quantum correction to the tunneling conductance is
shown to be proportional to flux-dependent "ballistic Cooperon". The obtained
results can be used for flux-tunable control of the magnetic disorder in
Aharonov-Bohm interferometers built on helical edge states.
