Dropdown switching subcategories portfolio

The ‘child_of’ argument lets you select subcategories from a parent category, and a you can get the current taxonomy id to pass as the value:

<?php 

$category_id = get_queried_object_id();
$args = array(
    'hide_empty'=>1,
    'depth'=>1,
    'hierarchical'=> 0, 
    'show_count'=> 1,
    'taxonomy'=> 'portfolio-types', 
    'child_of' => $category_id
    );
?>
<ul>
<?php wp_dropdown_categories( $args ); ?>
</ul>

So in the example above, only children (sub-categories) of portfolio-type of the current category ID will be displayed.

Note that this code will only work if you are in a taxonomy archive page, which I think that’s what you intend to do.