The software development profession suffers from severe gender biases, which could be explicit and implicit. However, SE literature has not systematically explored and evaluated the methods for reducing gender biases, especially for implicit gender biases. This paper reports on a field experiment to examine whether the intergroup contact theory could reduce implicit gender biases in software development. In the field experiment, 280 undergraduate students taking a project-centric introductory software engineering course were assigned to 70 teams with different contact configurations. We measured and compared their explicit and implicit gender biases before and after contacts in their teams. The study yields a rich set of findings. First, we confirmed the positive effects of intergroup contact theory in reducing gender biases, particularly the implicit gender biases in both general and SE-specific contexts. We further revealed that such effects were subjected to different contact configurations. The intergroup contact theory's effects were maximized in teams where the number of females is greater than or equal to the number of males. When the female is the minority group in a team, contacts among members contribute to reducing male members' implicit gender biases but fail to result in the same scale of effects on female members' implicit gender biases. The findings provide insights into using intergroup contact theory in reducing implicit gender biases in software development contexts.