  We consider a four-terminal Aharonov-Bohm interference setup formed out of
two edges of a quantum spin Hall insulator, supporting helical Luttinger
liquids (HLLs). We show that the temperature and bias dependence of the
interference oscillations are linked to the amount of spin flips in tunneling
between two HLLs which is a unique signature of a HLL. We predict that spin
dephasing depends on the electron-electron (e-e) interaction but differently
from the charge dephasing due to distinct dominant tunneling excitations. In
contrast, in a spinful Luttinger liquid with SU(2) invariance, uncharged spin
excitations can carry spin current without dephasing in spite of the presence
of e-e interactions.
