2 of 2 modified answer

Use linkText element Locator method instead of XPath.

    WebDriverWait wait = new WebDriverWait(driver, 10);
    WebElement element =wait.until(ExpectedConditions.presenceOfElementLocated(By.linkText("ABOUT US"))); 
   // WebElement aboutdrop=driver.findElement(By.linkText("ABOUT US"));

Also, try with removing driver.switchTo().defaultContent(); this line.

Generally switchTo().defaultContent(); method is used to go back on previous working window. So if it requires in your case then & then you may use.