Defendants charged with leading an armed occupation at a federal wildlife refuge in rural southeastern Oregon this year formally pleaded not guilty on Wednesday in a hearing saying little and sitting obediently — a stark contrast to the days when they drew world attention by broadcasting their view that the US government had illegally usurped lands across the West. The 25 men and women face up to six years in prison on charges that they impeded federal government employees from performing their duties in occupying the Malheur National Wildlife Refuge in a 40-day standoff.