Modify num items on author archive

wp_author_archive_limt

add_filter( 'pre_option_posts_per_page' , 'limit_posts_per_page');

function limit_posts_per_page()
{
    global $wp_query;
    if ( $wp_query->is_author=='1')
    {
        return 6;
    }
    else
    {
        $all_options = get_alloptions();
        return $all_options["posts_per_page"];
    }
}