A lawyer for Visa spent an entire court day Wednesday pummeling the sole expert witness put up by the Justice Department in the credit card antitrust case, an economics professor whom the card associations are ridiculing as an ivory tower-type who never worked in banking.

The professor, Michael L. Katz of the University of California at Berkeley, took the stand Tuesday and will likely stay there through Thursday - longer than any witness thus far in the case, which began June 12. Visa and MasterCard are spending hours trying to discredit the findings of a 226-page report he prepared in support of the government's position.

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