How to use Base Spark Components in Custom Vue Components?

Here is the code of my Custom Vue Component.

var base = require('settings/subscription/subscribe-braintree');

Vue.component('subscribe', {
    mixins: [base],

    /**
     * The component's data.
     */
    data() {
        return {
            plan: ''
        }
    }
});

I am using a Base Spark Component. But I am getting some error with a component used in the mixin of the base spark component. Where am I doing it wrong?

Here is the code of settings/subscription/subscribe-braintree

...

    mixins: [
        require('./../../mixins/braintree'),
        require('./../../mixins/plans'),
        require('./../../mixins/subscriptions')
    ],

...

        this.braintree('braintree-subscribe-container', response => {
            this.form.braintree_type = response.type;
            this.form.braintree_token = response.nonce;

            this.subscribe();
        });

...

I am getting the following error:

[Vue warn]: Error in mounted hook: "ReferenceError: braintree is not defined"