1. An echo canceller adapted for use in a communication system that includes a hybrid circuit, said echo canceller comprising: an adaptive digital filter that generates an estimated echo signal {circumflex over (z)}&lsqb;k&rsqb; in response to (i) a sampled input data sequence x&lsqb;k&rsqb; and (ii) an error signal sequence e&lsqb;k&rsqb; indicative of the difference between a near end signal sequence y&lsqb;k&rsqb; and the estimated echo signal {circumflex over (z)}&lsqb;k&rsqb;, wherein said adaptive digital filter computes filter coefficients based upon said error signal sequence e&lsqb;k&rsqb; using a stochastic quadratic descent estimator that employs a dynamically adjustable step size vector &mgr; &lsqb;k&rsqb; and said adaptive digital filter comprises means for computing said dynamically adjustable step size vector &mgr; &lsqb;k&rsqb; of the form 9 μ _ &af; [ k + 1 ] = μ _ &af; [ k ] + α &it; &it; φ _ &af; [ k ] &CenterDot; x _ &af; [ k ] &it; e &af; [ k ] &it; | μ min μ max, where &phgr; &lsqb;k&plus;1&rsqb;&equals; &phgr; &lsqb;k&rsqb;•( 1 − &mgr; &lsqb;k&rsqb;• x 2 &lsqb;k&rsqb;)&plus;e&lsqb;k&rsqb;)&plus;e&lsqb;k&rsqb; x &lsqb;k&rsqb; and &agr; is a scalar.