  Galactic Cosmic-ray (CR) transport parameters are usually constrained by the
boron-to-carbon ratio. This procedure is generically plagued with degeneracies
between the diffusion coefficient and the vertical extent of the Galactic
magnetic halo. The latter is of paramount importance for indirect dark matter
(DM) searches, because it fixes the amount of DM annihilation or decay that
contributes to the local antimatter CR flux. These degeneracies could be broken
by using secondary radioactive species, but the current data still have large
error bars, and this method is extremely sensitive to the very local
interstellar medium (ISM) properties. Here, we propose to use the low-energy CR
positrons in the GeV range as another direct constraint on diffusion models. We
show that the PAMELA data disfavor small diffusion halo ($L\lesssim 3$ kpc) and
large diffusion slope models, and exclude the minimal ({\em min}) configuration
(Maurin et al. 2001, Donato et al. 2004) widely used in the literature to
bracket the uncertainties in the DM signal predictions. This is complementary
to indirect constraints (diffuse radio and gamma-ray emissions) and has strong
impact on DM searches. Indeed this makes the antiproton constraints more robust
while enhancing the discovery/exclusion potential of current and future
experiments, like AMS-02 and GAPS, especially in the antiproton and
antideuteron channels.
