<A href='http://www.st.com/stonline/products/literature/ds/1773.pdf' Target=_Blank>External Web Link</a>
Bottom of page six.
Most H-bridge controllers have a convention such that when both inputs are at the same level and enabled, then low impedance (hence brake) will be applied to the motor with no voltage drive. This is true for the L298.
Take note of the warning about exceeding 2A during braking.