  We study the autocorrelation function of a conserved spin system following a
quench at the critical temperature. Defining the correlation length L(t)\sim
t^{1/z}, we find that for times t' and t satisfying L(t') << L(t) << L(t')^\phi
well inside the scaling regime, the spin autocorrelation function behaves like
<s(t)s(t')> = L(t')^{-(d-2+\eta)} [L(t')/L(t)]^{\lambda_c}. For the O(n) model
in the n -> \infty limit, we show that \lambda_c=d+2 and \phi=z/2. We give a
heuristic argument suggesting that this result is in fact valid for any
dimension d and spin vector dimension n. We present numerical simulations for
the conserved Ising model in d=1 and d=2, which are fully consistent with the
present theory.
