Preprint: <a href="https://arxiv.org/abs/cond-mat/9907176" rel="nofollow noreferrer">https://arxiv.org/abs/cond-mat/9907176</a> Mirror of published version since bactra.org may not be loading for you: <a href="https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=c2da22b83d499a916e71cbe20d18f23c2c8a19d9" rel="nofollow noreferrer">https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&d...</a>