How can I noindex my archive pages?

I am using the following snippet in my functions.php.

function add_noindex() {
    if(is_archive()) {
        echo 'meta name=robots content=noindex,follow /';
    }
}
add_action( 'wp_head', 'add_noindex' );

I thought this would noindex all my archive pages, but it is not working. My archive page code looks like this:

?php get_header(); ?
h1?php single_cat_title();?/h1

?php if (have_posts()) : while(have_posts()) : the_post();?
    h3?php the_title();?/h3
    ?php the_excerpt();?
    a href=?php the_permalink();?Read More/a
?php endwhile; 
endif;?

Any ideas? Thank you in advance!